1. Field of the Invention
The present invention relates to a method of dry-etching tungsten and a method of manufacturing a semiconductor apparatus which incorporates a gate electrode made of tungsten.
2. Description of the Related Art
Hitherto, polysilicon has widely been used as a material of a gate electrode of a semiconductor apparatus, such as a MOSFET. Since the degree of integration on the semiconductor apparatus and operation speed have been raised in recent years, polycide gates each having a two-layer structure constituted by a metal silicide film and a polysilicon film have been employed. Also self-align silicide gates have been employed which are formed by causing a metal film made of titanium or cobalt and formed on a polysilicon film and a polysilicon film to react with each other.
Pieces of semiconductor apparatus, such as MOSFET, in a generation incorporating a gate, the length of which is 0.13 xcexcm, or later are required to have a gate electrode exhibiting low resistance.
As gate electrodes each having resistance lower than that of the polycide gate and that of a selfalign silicide gate, polymetal gates having a laminate structure constituted by polysilicon, a reaction barrier and tungsten have attracted attention in recent years. Tungsten has specific resistance which is lower than that of tungsten silicide, which has been widely used, by one order. Therefore, use of tungsten as the material of the gate electrode enables time required to transmit a signal in the semiconductor apparatus to considerably be shortened. Hence it follows that the degree of integration and operation speed of the semiconductor apparatus can be raised.
Also research and development of dry etching which is a main technique for finely machining the polymetal gate have been performed from the above-mentioned technical viewpoint.
At present, use of mixed gas of chlorine (Cl2) and sulphur hexafluoride (SF6) is investigated as a gas for dry-etching tungsten because the mixed gas is time-tried gas and the gas exhibits easy handling. Sulphur hexauoride is decomposed in a plasma to generate fluorine radicals and fluorine ions in large quantities. Therefore, use of sulphur hexafluoride in a dry etching process causes a reaction W+6Fxe2x86x92WF6 ↑ to occur. Thus, tungsten can quickly be etched.
In a period in which dry etching is performed, accumulation of reaction products, reactions of etching seeds and desorption of the etching seeds are, from a microscopic viewpoint, quickly repeated in the surface of the film which must be etched. When the etching process is performed at relatively low speed, the surface of the film, which must be etched, has small number of asperities from a microscopic viewpoint. That is, the surface is flattened, as shown in FIG. 1. When the etching process is performed at relatively high speed, the surface of the film, which must be etched, has great asperities from a microscopic viewpoint, that is, the surface is roughened, as shown in FIG. 2.
Higher-order fluorine gas, such as sulphur hexafluoride, generates fluorine in a large quantity. Therefore, use of sulphur hexafluoride to serve as the gas for etching tungsten raises the etching rate of tungsten. Simple rise in the etching rate does not raise critical problem from a viewpoint of etching tungsten. When a thin film made of materials which do not have satisfactory selectivity with tungsten and etching is formed below the tungsten, asperities formed on the surface owing to the process for dry-etching tungsten exerts an adverse influence on the actions of the materials of the lower layer. The pieces of semiconductor apparatus, such as the MOSFET, in the generation having the length of 0.13 xcexcm, or later have the structure that the thickness of the gate insulating film is several nm order or smaller. Therefore, there is apprehension that the gate insulating film is broken because asperities formed during etching of tungsten reach the gate insulating film if the degree of the asperities is too large.
Ions which are involved in the etching reaction can be controlled by controlling the width of distribution of incident particles in the outermost surface of the substrate with a bias applied to the electrode provided for the substrate. Since radicals which are involved in the etching reaction are not charged particles, the width of distribution of incident particles in the outermost surface of the substrate cannot easily be controlled. When etching gas containing radicals in a relatively small quantity with respect to ions is employed, the etching rate in a region in which hairlines are formed densely and that in a region in which hairlines are isolated from one another are substantially the same, as shown in FIG. 3. When etching gas containing radicals in a relatively large quantity with respect to ions is employed, a microloading effect exerts an adverse influence. Thus, the etching rate is reduced in the region in which the hairlines are formed densely and the etching rate is raised in the region in which hairlines are isolated from one another, as shown in FIG. 4.
In general, fluorine requires a long time from generation of fluorine radicals to the demise of the same in a plasma. Therefore, the quantity of generated fluorine radicals is enlarged in a case of gas, such as sulphur hexafluoride, containing fluorine in a large quantity in the gas molecule. When the higher-order fluorine gas, such as sulphur hexafluoride, is employed as the gas for dry-etching tungsten, the quantity of radicals in the plasma is enlarged relatively. As a result, the difference in the mask pattern in the same chip inhibits realization of a uniform etching rate. Hence it follows that satisfactory shape controllability cannot be obtained.
In view of the foregoing, an object of a present invention is to provide a method of dry-etching tungsten with which satisfactory shape controllability can be obtained by flattening the etched surface.
Another object of the present invention is to provide a method of manufacturing a semiconductor apparatus with which satisfactory shape controllability can be obtained and which is capable of forming a gate electrode constituted by laminating tungsten without any breakage of a gate insulating film.
According to one aspect of the present invention, there is provided a dry etching method comprising the step of dry-etching tungsten with mixed gas containing fluorine gas and chlorine or hydrogen bromide, oxygen and nitrogen.
According to another aspect of the present invention, there is provided a method of manufacturing a semiconductor apparatus comprising the steps of: laminating upwards a polycrystal silicon film or an amorphous silicon film, a tungsten nitride film or a titanium nitride film and a tungsten film on a silicon substrate; and dry-etching the tungsten nitride film or the titanium nitride film and the tungsten film with mixed gas containing fluorine gas and chlorine or hydrogen bromide, oxygen and nitrogen so that a gate electrode is formed.
The dry etching method according to the present invention is arranged to dry-etch tungsten with the mixed gas and form dry-etched tungsten into a predetermined shape.
The method of manufacturing a semiconductor apparatus according to the present invention has the steps of sequentially and upwards forming either of the polycrystal silicon film or the amorphous silicon film and forming the tungsten nitride film or the titanium nitride film so that a tungsten film is formed. Then, the method of manufacturing a semiconductor apparatus according to the present invention has the step of dry-etching the tungsten nitride film or the titanium nitride film and the tungsten film with the mixed gas so that a gate electrode is formed. Note that the tungsten nitride film and the titanium nitride film are formed between the polycrystal silicon film or the amorphous silicon film and the tungsten film to serve as a barrier layer. Therefore, the thickness of each the tungsten nitride film and the titanium nitride film can be reduced with respect to the tungsten film.
The mixed gas contains the fluorine gas, gas of either of chlorine or hydrogen. bromide, oxygen and nitrogen.
The functions of the gases contained in the mixed gas will now be described.
The fluorine gas is, in a plasma, decomposed into fluorine radicals and fluorine ions which are combined with tungsten to enhance etching.
The fluorine gas according to the present invention has a structure that the ratio of fluorine atoms with respect to elements of the gas molecule except for fluorine is four or less. When the composition of the fluorine molecule is MXFY, fluorine gas having a ratio Y/X of four or less is employed, where M is an element except for fluorine atom and F is fluorine. Specifically, fluorine gas is employed which has a structure that the total number of fluorine atoms in elements which constitute the gas molecule, such as CF4, CHF3, CH2F2 or CHF3, is four or less. The fluorine gas containing fluorine atoms at a ratio of four or less realizes an etching rate which is slower than that realized by higher-order fluorine gas, such as sulphur hexafluoride. Therefore, a practical etching rate with respect to a tungsten film can be realized. The fluorine gas containing fluorine atoms at the ratio of four or less generates fluorine. radicals in a quantity which is relatively smaller than that of fluorine ions. Therefore, the influence of the microloading effect can be reduced.
The foregoing mixed gas contains either of the chlorine gas or bromine gas. Chlorine gas forms oxy halide (WOXClY) of tungsten in association with oxygen to enhance etching of tungsten. Similarly, bromine forms oxy halide (WOXBrY) in association with oxygen to enhance etching of tungsten.
Nitrogen nitrides side walls of the tungsten film and the like to form tungsten nitride to prevent progress of etching in a region in which hairlines are isolated from one another. Therefore, nitrogen makes the etching rate in a region in which hairlines are formed densely and that in a region in which Hairlines are isolated from one another to be uniform.
Since the mixed gas contains the fluorine gas, the gas of chlorine or hydrogen bromide, oxygen and nitrogen, the microloading effect can be prevented. Moreover, tungsten can be etched at a practical rate.